Here are the classes defined in Collection Interface in java.util:
ArrayList Class: ArrayList Class handles the dynamic array and List interface in java.util. It extends AbstractList class. ArrayList is actually an array of variable length which can be increased or decreased in size dynamically.
LinkedList Class: LinkedList Class handles the linked list and List interface in java.util. It extends AbstractSequentialList class.
HashSet Class: HashSet Class is used to create a hash table and implements Set interface in java.util. It extends AbstractSet class.
LinkedHashSet Class: LinkedHashSet Class introduces a linked list order in the HashSet Class by extending it. It has no members of its own.
TreeSet Class: TreeSet Class is used to store objects in a sorted ascending order. TreeSet provides fast access and retrieval of objects.
AbstractCollection class: AbstractCollection class is used to implement Collection interface.
AbstractList class: It extends AbstractCollection class and is used to implement List interface.
AbstractSequentialList class: It extends AbstractList class and is used by a collection that uses sequential access of its elements.
AbstractSet class: It extends AbstractCollection class and is used to implement Set interface